An OLED (organic light-emitting diode) is a newly developed flat panel display device, and has a broad application prospect due to that it has advantages such as self-luminescence, high contrast, wide color gamut, simple manufacturing process, low cost, low power consumption, being easy to implement flexible display, ant the like.
An OLED pixel circuit in an organic electroluminescent display device is generally arranged in a matrix. OLED pixel circuits can be classified into two types of passive matrix organic light emission display (PMOLED) pixel circuit and active matrix organic light emission display (AMOLED) pixel circuit according to drive modes thereof. Although the PMOLED has advantages such as simple process and low cost, it cannot meet the requirements of high-resolution large-size display due to disadvantages such as crosstalk, high power consumption, short service life, and the like. In contrast, in the AMOLED, each pixel circuit is integrated with a set of thin film transistors (TFTs) and a storing capacitor (simply referred to as CS) therein, and an electric current flowing through the OLED is controlled by controlling the drive on the thin film transistors TFT and the storing capacitor CS to make the OLED luminous. As compared with the PMOLED, the AMOLED can meet the requirements of large-size display with a high resolution and multiple grayscales due to its small drive current, low power consumption, and long service life. Further, the AMOLED has obvious advantages in terms of viewable angle, color restoration, power consumption, response time, and the like, and is applicable to a display device with high information content and a high resolution.
FIG. 1 is a schematic diagram showing the structure of an AMOLED pixel circuit of 4T1C (four transistors and one capacitor) type in the prior art, wherein an electric current flowing though the OLED is as follows:IOLED=IT1=k(VDATA−NTH)2  (1)
In the Equation (1), k is a constant relevant to the structure of T1, VDATA is a data voltage, and VTH is a threshold voltage of the T1.
Since the OLED is a device driven by an electric current, from the Equation (1), it can be seen that the electric current flowing through the OLED is not only controlled by the data voltage VDATA, but also influenced by the threshold voltage VTH of TFT. Thus, the structure of the OLED pixel circuit as shown in FIG. 1 cannot compensate drift and inconsistency of the threshold voltage of the TFT, and thus threshold characteristics of the TFT affect a drive current greatly. Further, during manufacture of an array substrate, since the manufacturing process of an oxide TFT is not mature enough, characteristics of the oxide TFT, such as threshold voltage and mobility, varies greatly in different regions, and thus TFTs in various OLED pixel circuits cannot have completely consistent performance parameters. At the same time, as time goes on, a threshold of each TFT will shift and a drive current of each TFT will change due to the presence of a voltage stress, resulting in that electric currents flowing through OLEDs in various OLED pixel circuits are inconsistent. Thus, brightness of light emitted by various OLED pixel circuits is nonuniform, which affects the brightness of the final display greatly. Therefore, the brightness of the whole display screen is nonuniform, which affects the display effect.